The invention relates to an adhesive comprising two components A and B, component A having at least two different functional groups and component B having at least two similar functional groups.
The mechanical production of composite materials, especially composite films, is frequently conducted in the art by lamination using solvent-based adhesives. From a variety of standpoints, however, such a procedure is disadvantageous.
When using solvent-based adhesives in lamination, for example, it is necessary to evaporate considerable volumes of solvent during the laminating process, involving high energy consumption. A further necessity, for the purpose of avoiding the emission of solvent vapors into the atmosphere, is extensive cleaning of the waste air produced during evaporation of the solvent. In recent times, therefore, the tendency when producing composite materials has been more and more toward using solventless systems rather than those containing solvent.
However, the renunciation of solvents impacts very adversely on the processability of an adhesive. Adhesives suitable for producing composite materials are intended first to have a suitable processing viscosity but, as far as possible, to release only small volumes of volatiles into the environment. A further requirement of such adhesives is that directly after application to at least one of the materials to be joined, and after joining thereof, they possess sufficiently good initial adhesion to prevent, as far as possible, the bonded materials from moving relative to one another. Furthermore, however, an adhesive bond of this kind should also possess a sufficient degree of flexibility to withstand the various tensile and extension stresses to which the composite material is generally exposed while still at the processing stage, and to do so without damage to the adhesive bond and without damage to the bonded material.
With the prior art, conventional, solvent-free adhesives, therefore, there is generally a fundamental disadvantage in that the adhesion properties of the adhesive following application are unsatisfactory, owing to the low viscosity, and, accordingly, the adhesive bond may not be subjected to any stresses until it has finally cured, in order that the composite material maintains the form intended by adhesive bonding. This, however, necessitates long curing times, which frequently negate the economy of composite materials produced using such adhesives.
One approach to avoiding the disadvantages described above was to use a multistage-curing adhesive system in the production of composite materials. Adhesives were used which in a first stage were subjected to a rapid, first curing reaction by irradiation. The strength of the adhesive bond following this first curing reaction was said to be such as to enable the bonded articles or materials to be handled without problems. In a second curing stage, the adhesive then cured further until it had reached the desired ultimate strength.
DE-A 29 13 676 discloses a process for producing composite films by means of solvent-free adhesives. It describes a solvent-free adhesive which is liquid at room temperature and consists of oligomeric and/or polymeric esters and/or ethers which contain both free isocyanate groups and free (meth)acrylate groups in one molecule.
EP-B 0 564 483 relates to reactive contact adhesives, processes for preparing them, and their use. The document describes urethane-based coating compositions which are polymerizable in two stages and which by virtue of the presence of UV-polymerizable acrylate groups can be cured in a first curing stage to a material which has solidified but cannot yet be embossed or deformed in such a manner as to impart structure, after which, in a second, subsequent stage, irreversible solidification takes place. To reduce the viscosity, monofunctional acrylates are added to the adhesive. Following irradiation, the adhesive described exhibits pressure tack; the intended use of the contact adhesive described is said to be the adhesive bonding of wood and/or plastics parts at up to about 70° C., preferably at room temperature.
In many cases, composite materials are subjected directly after their production to further processing operations which demand a high degree of flexibility from the composite material. The adhesive bond present in the composite material must therefore possess sufficient flexibility to permit its further processing, without adverse effects owing to a lack of flexibility of the adhesive bond. For this purpose it is necessary for the adhesive bond to possess, initially, sufficient strength to prevent separation of the composite material into its original constituents, but not yet to be so strong that adverse effects owing to an excessively strong adhesive bond arise under tensile or flexural stress.
The adhesives known from the prior art have the disadvantage that they exhibit either excessively strong initial adhesion, which has a deleterious effect on the flexibility of the material, or else inadequate ultimate strength, which may be disadvantageous for the service properties of the composite material. Furthermore, the reactive diluents described in the prior art are frequently not fully reacted. This may lead to odor nuisance and even, if appropriate, to a health risk owing to migrateable compounds of low molecular mass (migrants).